Lubricant base stocks are one of the higher value products that can be generated from a crude oil or crude oil fraction. The ability to generate lubricant base stocks of a desired quality is often constrained by the availability of a suitable feedstock. For example, most conventional processes for lubricant base stock production involve starting with a crude fraction that has not been previously processed under severe conditions, such as a virgin gas oil fraction from a crude with moderate to low levels of initial sulfur content.
One of the challenges in producing lubricant base stocks can be to generate a desirable yield of high performance base stocks, such as Group III base stocks, Group II heavy neutral base stocks, and/or Group II bright stocks. Conventionally, catalytic hydroprocessing can be used to generate lubricant base stocks with a high viscosity index, but the yield of base stocks with heavy neutral or brightstock viscosity can tend to be reduced or minimized. Conventionally, solvent processing can be used to generate lubricant base stocks with a high viscosity, but such base stocks produced by solvent processing can tend to correspond to Group I base stocks.
U.S. Pat. No. 6,592,748 describes methods for raffinate hydroprocessing. A lubricant boiling range feed, such as a light or medium neutral vacuum distillate feed, is solvent extracted. The raffinate from solvent extraction is then mildly hydroconverted and dewaxed. Production of Group II base stocks having various viscosities is described, such as viscosities up to about 7 cSt at 100° C.
U.S. Pat. No. 8,394,255 describes methods for forming lubricant base stocks using catalytic hydroprocessing. The methods include performing dewaxing under sour conditions during base stock production.